1. Technical Field to Which the Invention Belongs
This invention relates to a novel compound and an organic polymer, a resin composition containing them, and nonlinear optical device prepared using the resin composition.
2. Prior Art
Nowadays, materials having nonlinear optical activities (NLO materials) sufficiently highly effective to double or triple the frequency of electromagnetic waves attract great scientific and technical interest as materials usable in optical long-distance communication, signal processing, photoelectric hybrid circuits and optical computers.
Nonlinear optics is concerned with the mutual action of electromagnetic waves in various mediums, which is necessary for generating a new field variable in phase frequency or amplitude, and electro-optic adjustment of electromagnetic waves is represented by, e.g., the following two expressions: EQU .DELTA.n=-(1/2)n.sup.3 r.sub.33 E EQU r.sub.33 .varies..beta..mu.
In the above, .DELTA.n is change in refractive index, n is refractive index, r.sub.33 is electro-optic constant, E is electric-field strength, .beta. is secondary nonlinear susceptibility, and .mu. is dipole moment. The r.sub.33 increases with an increase in .beta..mu., and therefore .DELTA.n increases with .beta..mu..
As a compound having a secondary nonlinear susceptibility, having a vinylene group between an aromatic ring and a pyran ring, Van Aran et al. discloses 4-(dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (hereinafter "DCM") in Japanese Patent Application Laid-open No. 54-21722 and U.S. Pat. No. 4,145,215.
The DCM, however, has so poor a stability to light or heat that, although it has been possible to prepare an optical device using a secondary nonlinear susceptibility, actually it has been difficult to ensure drive reliability for a time long enough to withstand practical use. For example, Appl. Phys. Lett., 61, pp.2272-2274 (1992) reports a method in which an organic polymer composition having a nonlinearity is produced by a guest-host system where the DCM is added in polyimide, and an electro-optical device constituted of an optical waveguide is fabricated using this composition. In addition thereto, as a chemically modified product of the above compound, having a high thermal stability, PROC. SPIE, 1853, pp.183-192 (1993) also presents a compound having a cyclohexene ring in place of the pyran ring. Chem. Mater., 9(6), pp.1437-1442 (1997) reports some derivatives each having a cyclohexene ring which have a nonlinear ability. In this report, synthesis of novel compounds in which an aromatic ring and a pyran ring are coalesced is reported, but there is no description on the nonlinearity and stability of the compounds.
In the case of the guest-host system utilizing the addition of a nonlinear compound to an organic polymer, there has been a problem that the nonlinear optical characteristics may deteriorate because the nonlinear compound thus added disappears because of thermal diffusion occurring under high-temperature conditions when the composition is cured by heating or when optical devices made from the composition are subjected to annealing or soldering. For example, Appl. Phys. Lett., 61, pp.2272-2274 (1992) reports that 220.degree. C. is the limit in the case of the DCM. Also when the nonlinear compound is kept oriented by a suitable method, there has been a problem that its treatment at a high temperature may make the orientation become loose by heat to cause deterioration of nonlinear optical characteristics. Still also when the composition is prepared by adding the nonlinear compound at a high concentration in a polymer solution, there has been a problem that the compound may deposit because of its crystallization or insolubilization at the time of solvent evaporation or heat curing, to cause a deterioration of film properties and a great propagation loss.